Superheterodyne receiver

A superheterodyne receiver (often shortened to superhet) is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. It was invented by US engineer Edwin Armstrong in 1918 during World War I. Virtually all modern radio receivers use the superheterodyne principle.

"Superheterodyne" is a contraction of "supersonic heterodyne", where "supersonic" indicates frequencies above the range of human hearing. The word heterodyne is derived from the Greek roots hetero- "different", and -dyne "power". In radio applications the term derives from the "heterodyne detector" pioneered by Canadian inventor Reginald Fessenden in 1905, describing his proposed method of producing an audible signal from the Morse code transmissions of the new continuous wave transmitters. With the older spark gap transmitters then in use, the Morse code signal consisted of short bursts of a heavily modulatedcarrier wave, which could be clearly heard as a series of short chirps or buzzes in the receiver's headphones. However, the signal from a continuous wave transmitter did not have any such inherent modulation and Morse Code from one of those would only be heard as a series of clicks or thumps. Fessenden's idea was to run two Alexanderson alternators, one producing a carrier frequency 3 kHz higher than the other. In the receiver's detector the two carriers would beat together to produce a 3 kHz tone thus in the headphones the Morse signals would then be heard as a series of 3 kHz beeps. For this he coined the term "heterodyne" meaning "generated by a difference" (in frequency).

The superheterodyne principle was devised in 1918 by U.S. Army Major Edwin Armstrong in France during World War I. He invented this receiver as a means of overcoming the deficiencies of early vacuum tube triodes used as high-frequency amplifiers in radio direction finding equipment. Unlike simple radio communication, which only needs to make transmitted signals audible, direction-finders measure the received signal strength, which necessitates linear amplification of the actual carrier wave.

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